Various methods are known for microfibrillating plant fibers or the like (e.g., wood pulp) to obtain microfibrillated plant fibers (nanofibers) that are refined to have a nano-order fiber diameter. For example, Patent Literature 1 discloses that by microfibrillating a cellulose fiber having a specific fiber length, a microfibrillated cellulose having excellent water retentivity and a long fiber length can be obtained despite a small fiber diameter. Patent Literature 2 suggests a method for enhancing nanofibrillation, in which the adhesive property of unnecessary lignin, hemicellulose, and the like, contained in a cellulose-based fiber raw material is diminished by subjecting the fiber raw material to steaming treatment. Further, as a method for directly producing a cellulose nanofiber from lignocellulose by enhancing nanofibrillation, Patent Literature 3 suggests a method for treating lignocellulose in an aqueous-based medium containing a nitroxy radical derivative, alkali bromide, and an oxidizing agent.
Patent Literature 4 discloses a method for improving the water absorption property of fiber for use in disposable diapers and the like, in which a hydrophobized drug such as an anionic surfactant, cationic surfactant, or nonionic surfactant is added to a cellulose-based fiber, and then the mixture is subjected to mechanical stirring to provide the cellulose-based fiber with a high porosity. As in Patent Literature 4, although the production of microfibrils as small as microfibrillated plant fibers (nanofibers) is not intended, Patent Literature 5 suggests increasing the affinity with an anionic dye by introducing a cationic group into the surface of a cellulose-based fiber to cationically charge the fiber surface, and improving the water retentivity, shape retention property, and dispersibility of cellulose particles, while maintaining functions as the cellulose particles obtained by further refining the cellulose-based fiber.